Yevamos4: 9
שׁוֹמֶרֶת יָבָם שֶׁקִּדֵּשׁ אָחִיו אֶת אֲחוֹתָהּ — מִשּׁוּם רַבִּי יְהוּדָה [בֶן בְּתֵירָא] אָמְרוּ: אוֹמְרִים לוֹ, הַמְתֵּן עַד שֶׁיַּעֲשֶׂה אָחִיךָ הַגָּדוֹל מַעֲשֶׂה. חָלַץ לָהּ אָחִיו, אוֹ כְנָסָהּ — יִכְנוֹס אֶת אִשְׁתּוֹ. מֵתָה הַיְבָמָה — יִכְנוֹס אֶת אִשְׁתּוֹ. מֵת יָבָם — יוֹצִיא אֶת אִשְׁתּוֹ בְגֵט, וְאֵשֶׁת אָחִיו בַּחֲלִיצָה.
[If] a woman [was] awaiting her yavam, and his brother then betrothed her sister, it was said in the name of R' Yehudah [ben Beseira]: We tell him, “Wait until your oldest brother acts!” [If] his brother performed chalitzah with her or married her, he may consummate the marriage with his wife. [If] the yevamah died, he may consummate the marriage with his wife. [If] the yavam died, [however,] he must discharge his wife with a bill of divorce, and his brother's wife with chalitzah.
Yevamos4: 10
הַיְבָמָה לֹא תַחֲלוֹץ וְלֹא תִתְיַבֵּם עַד שֶׁיֶּשׁ לָהּ שְׁלֹשָׁה חֳדָשִׁים. וְכֵן כָּל שְׁאָר הַנָּשִׁים לֹא יִתְאָרְסוּ וְלֹא יִנָּשְׂאוּ עַד שֶׁיִּהְיוּ לָהֶן שְׁלֹשָׁה חֳדָשִׁים — אֶחָד בְּתוּלוֹת וְאֶחָד בְּעוּלוֹת, אֶחָד גְּרוּשׁוֹת וְאֶחָד אַלְמָנוֹת, אֶחָד נְשׂוּאוֹת וְאֶחָד אֲרוּסוֹת. רַבִּי יְהוּדָה אוֹמֵר, הַנְּשׂוּאוֹת יִתְאָרְסוּ, וְהָאֲרוּסוֹת יִנָּשְׂאוּ, חוּץ מִן הָאֲרוּסוֹת שֶׁבִּיהוּדָה, מִפְּנֵי שֶׁלִּבּוֹ גַּס בָּהּ. רַבִּי יוֹסֵי אוֹמֵר, כָּל הַנָּשִׁים יִתְאָרְסוּ, חוּץ מִן הָאַלְמָנָה מִפְּנֵי הָאִבּוּל.
The yevamah may not perform chalitzah or be taken in yibum until three months have passed. Similarly, all other women may not be betrothed or married until three months have passed —- be they virgins or nonvirgins, divorced or widowed, married or betrothed. R' Yehudah says: Those who were married may be betrothed, and those who were betrothed may be married, except for those who were betrothed in Judea, since he is familiar with her. R' Yose says: All women may be betrothed, except for the widow because of [her] mourning.